As environmental concerns have led to increasingly strict regulation of engine emissions by governmental agencies, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important. Current indications are that this trend will continue.
In the past, the emission levels of US diesel engines have been regulated according to the Environmental Protection Agency (EPA) using the Federal Test Procedure (FTP) cycle, with a subset of more restrictive emission standards for California via the California Air Resources Board (CARB).
Future emission from diesel engines will have to be further reduced in order to meet proposed and soon to be implemented EPA emission standards. For example, the Tier II emission standards, which are being considered for 2004, are 50% lower than the Tier I standards. Car and light truck emissions are measured over the FTP 75 test and expressed in gm/mi.
Regulatory agencies continue to propose and apply ever-stricter emission standards. For example, proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/mi NOx and 0.08 gm/mi particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and ULEVs in California would have to meet a 0.05 gm/mi NOx standard to be phased in over a three-year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/mi would also have to be met. The EPA has also proposed tighter regulations for off-road diesel engines requiring them to emit 90% less particulate matter and nitrogen oxides by 2014 than they do today.
Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping, by themselves, will not be able to achieve the low emission levels required by these standards. Aftertreatment technologies will have to be used and will have to be further developed in order to meet the future low emission requirements set for diesel engines.
Some promising aftertreatment technologies to meet future NOx emission standards include lean NOx catalysts, NOx adsorbers, and Selective Catalytic Reduction (SCR) catalysts. Currently, used lean NOx catalyst technologies result in the reduction of engine NOx emissions in the range of 10 to 30 percent for engines operated under typical conditions. Although a promising technology, SCR catalyst systems require an additional reducing agent (aqueous urea). The need for this compound raises issues related to the relatively high freezing point of the compound and the need to develop and support a distribution system for this compound.
When NOx adsorbers are used to sequester NOx they must be periodically regenerated. One way of regenerating NOx adsorbers is by using pre-cats (catalysts, which partially oxidize hydrocarbon to produce reductants and heat). Commonly used pre-cats produce exhaust gasses enriched in volatile hydrocarbons, CO2, and water. These compounds are effective at regenerating commonly used NOx adsorbers when the adsorbers are regenerated at temperatures in the 500° C. range. The need for elevated temperatures make this class of reductants impractical for the regeneration of NOx adsorbers used with internal combustion engines that operate at relatively low temperatures, such as, light duty diesel engines. Light-duty diesel engines are commonly found in cars and light duty trucks, a rapidly growing segment of the diesel engine market.
Another promising approach is the use of a non-catalytic process for the removal of NOx and particulates from engine exhausts. Oxygen rich diesel engine exhaust containing NOx is fed into a plasma generator incorporating for example a gamma-aluminum component. Electrical current and additional hydrocarbon fuel supplied to the unit are used to produce volatile hydrocarbons that react with NOxs and carbon-based soot in engine exhaust to produce more environmentally benign products such as N2 and CO2. For a more comprehensive discussion of this technology the reader is directed toward U.S. Pat. No. 6,038,854 to Penetrante, et al. herein incorporated by reference in its entirety. The primary component of the non-catalytic NOx removal system is a plasma source requiring a continuous source of electrical current, therefore the use of this system may result in a significant fuel penalty.
Non-thermal plasma generators use electrical current and oxygen, and operating at temperatures in the range of 500° C. These devices reform hydrocarbons to produce reductants enriched in reactive oxygenated organic molecules. Ready source of hydrocarbon fuel includes, for example, diesel fuel. Reactive oxygenates produced by the process can react with NOx and carbon-based soot to produce environmentally benign species such as N2, CO2, and H2O. For a more detailed discussion of this technology the reader is directed to U.S. Pat. No. 6,176,078 to Balko et al., and to “Thermal Cracking of Higher Paraffins” by H. H. Voge and G. M. Good, Journal of the American Chemical Society, Vol. 71, pages 593-597, February, (1949).
The oxygenated organic molecules produced by this system contain at least one carbon atom and generally no more than 3 carbons. The longer chain reductants may have difficulty permeating ultra-fine NOx adsorber matrices or heavily sooted particulate filters.
Internal combustion engine exhaust gas aftertreatment systems that use plasma fuel converters, which require a supply of fresh air, water, fuel, and electricity, have been used to produce reductants, which are used, in turn, to regenerate NOx adsorbers. See, for example, U.S. Pat. No. 6,560,958 to Bromberg. The systems proposed so far require a dedicated source of water and air to ensure the efficient operation of the plasma fuel generator. The need for a dedicated source of water limits the utility of these systems, especially when they are used with mobile internal combustion engines or with stationary engines operated in environments which lack ready access to a dedicated water supply.
These technologies, therefore, have limitations that may prevent their use in achieving the new emissions requirements as efficiently as possible. There is a need then for an engine aftertreatment system that provides a source of extremely reactive reductants that can effectively regenerate NOx adsorbents, including systems using an ultra-fine catalyst bed, that does not result in a significant fuel penalty and that can be readily operated in the absence of a dedicated supply of fresh air and water. The present invention is directed toward meeting this need.